CeReS (Cell Regulatory Sialoprotein) is a molecule demonstrated to: 1. Inhibit primary, continuous and transformed cell lines, 2. Affect cells from species as diverse as insects and humans, 3. Cause one cell line to terminally differentiate, 4. Reversibly inhibit other lines (90%-98% of the cells recover), 5. Specifically arrest cells near the Gl/S interface, 6. Synchronously release cells to enter S phase and then mitosis (synchronization remains through at least one additional cell cycle). CeReS is distinct from all known drugs and cell surface components functionally and according to the limited peptide sequence and preliminary DNA sequence data available. The aims of this study are to determine the feasibility of using CeReS to improve and increase efficiency of cell culture, to provide new models for differentiation; and to test a wide variety of cancer cell lines for sensitivity or resistance to CeReS. Preliminary data indicates differential sensitivity of cells suggesting potential therapeutic uses. SynchroCell, the exclusive licensee of CeReS, believes this and related molecules will be important products for the research community, significantly aiding in understanding regulation and carcinogenesis and as a cell culture tool of commercial importance. Improved cancer therapies are potential applications of CeReS and of the knowledge gained with the aid of CeReS.